Correspondence of the functional epitopes of poxvirus and human interleukin-18-binding proteins

Molluscum contagiosum virus, a human poxvirus that causes persistent small benign skin tumors, encodes a variety of putative immune defense proteins. Three such proteins, MC51L, MC53L, and MC54L, have 20 to 35% amino acid seq uence identities with human interleukin-18 (hIL-18)-binding protein (hIL-18 BP), a naturally occurring antagonist of the proinflammatory cytokine IL-18 . We previously demonstrated that seven amino acids within the immunoglobul in-like domain of hIL-18BP were important for high-affinity binding to hIL- 18. Model building indicated that MC54L, which has been shown to bind hIL-1 8, contains five of the seven amino acids at corresponding positions in its immunoglobulin-like domain, the exceptions being the conservative substitu tion of isoleucine for a leucine and the nonconservative substitution of va line for a phenylalanine. We found that individual alanine substitutions fo r these six identical or highly conserved amino acids of MC54L caused chang es in affinity and binding free energy for hIL-18 that were quantitatively similar to those produced by mutagenesis of hIL-18BP. Furthermore, when the nonconserved valine of MC54L was mutated to phenylalanine, making it more like hIL-18BP, its affinity for hIL-18 increased more than 10-fold. In addi tion, the carboxyl-terminal half of MC54L, which has no similarity with ML- 18BP, was dispensable for hIL-18 binding. Thus, despite their relatively lo w overall sequence identity, MC54L and hIL-18BP have similar hIL-18 binding sites and functional epitopes. On the other hand, MC51L and MC53L have non conservative substitutions of three to six of the seven critical amino acid s of hIL-18BP and neither protein bound hIL-18, suggesting that they may in teract with unidentified ligands.